1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming which is capable of performing an improved circulation of developer.
2. Description of the Related Art
FIG. 1 generally illustrates an example of an image forming apparatus such as a copier, a facsimile, a printer, etc. The illustrated example of the image forming apparatus is an image forming apparatus 2 which includes a photosensitive medium rotating unit 4, a toner feeding unit 38, a transfer unit 8, a fixing unit 10, paper feeding cassettes, etc. The photosensitive medium rotating unit 4 includes an image creating device such as a photosensitive medium 15 as an image carrier, and a developing unit 16 for developing an electrostatic latent image formed on a photosensitive medium with a toner. These two units may be integrally or separately formed. A copy sheet fed from a desired paper feeding cassette is transported to a pair of registration rollers 17 by transport rollers or the like, transported to the transfer unit 8 with timing coincident with a toner image formed on the photosensitive medium 15, and receives the toner image transferred thereto. The copy sheet carrying an unfixed toner image is transported to the fixing unit 10, where the toner image is fixed, and then is ejected.
FIG. 2 schematically illustrates a main portion of the developing unit 16. The developing unit 16 is provided therein with a developer carrier 41, a developer regulating member 42, a developer stirring member 43, etc. A so-called doctor blade is one example of the developer regulating member 42. The developer carrier 41 has a rotatable non-magnetic sleeve 45 as a main body, and a magnetic field generating member 44 is fixedly arranged inside the sleeve 45. The developer carrier 41 is rotated in a direction indicated by an arrow A while carrying a developer (for example, a two-component developer consisting, for example, of a carrier and a toner, hereinafter simply called the "developer") which is magnetized by the action of a magnetic field generated by the magnetic field generating member 44 to transport the developer to a developing position.
In this event, the developer regulating member 42 regulates the amount of developer (developer holding amount) carried by the developer carrier 41. More specifically, the thickness of the developer carried on the developer carrier 41 and transported to the developing position is regulated by the developer regulating member 42. The developer stirring member 43 stirs and transports the developer within the developing unit 16.
FIG. 3 is a conceptual diagram illustrating the relationship between a magnetic field generated by the magnetic field generating member 44 including a plurality of magnetic poles (P1-P6) disposed within the developer carrier 41 and the position of the developer regulating member 42. Lines forming oval figures indicate a distribution of the magnitude of the magnetic field generated around the developer carrier 4 1, in the direction of the normal to the circumferential surface of the developer carrier (in this specification, simply referred to as the "normal magnetic flux density"). A larger extension of the oval figures in the radial direction of the developer carrier 41 represents a higher normal magnetic flux density at the respective rotating angular positions relative to the magnetic poles P1-P6 of the magnetic field generating member 44.
The developer contained in the developing unit 16 is drawn up by a draw-up pole P5 of the developer carrier 41 on the developer carrier 41 and held thereon. The drawn developer moves in accompaniment of the rotation with the developer carrier 41 in the direction A, such that the amount of the developer held on the developer carrier 41 is regulated to be constant by the developer regulating member 42 (in other words, the thickness of the developer on the developer carrier 41 is ideally constant after it passes the position at which the developer regulating member 42 is disposed). The developer having passed the developer regulating member 42 visualizes (or develops) an electrostatic latent image on an image carrier at the developing position. The developer having a reduced toner concentration due to the development is further transported in accompaniment with the rotation of the developer carrier 41 to again return into the developing unit 16. Then, the developer is separated from the developer carrier 41 by the action of a developer separating pole P4, and delivered toward the developer stirring member 43. When the amount of developer drawn up by the pole P5 exceeds a predetermined developer holding amount, an excessive portion of the developer is regulated by the developer regulating member 42 and therefore cannot pass the same, and such portion is removed from the developer carrier 41. The developer removed from the developer carrier 41 is separated from the developer carrier 41 as it is led by the surface of the developer regulating member 42, and drops due to its own weight at a point where the magnetic field does not act sufficiently thereon as indicated by an arrow C in FIG. 2.
Conventionally, the developer regulating member 42 is often positioned downstream of a rotating angular position at which a normal component of the magnetic field becomes zero (in this specification, called the "normal magnetic flux density inflection point") in the direction in which the developer carrier is rotated (this direction is hereinafter called the "developer carrier rotating direction). This is because the following inconveniences result from the regulation of the developer holding amount made at a position where the normal magnetic flux density is increased or at a position where the normal magnetic flux density inflection point exists. Specifically, if the developer holding amount is regulated at a position where the normal magnetic flux density is increased, i.e., where the magnetic field curve extends furthest away from the developer carrier 41 in the form of lobe (in the example of FIG. 3, where the solid line is largely separated from the developer carrier), the developer carrier 41 would have a large torque. Conversely, if the developer holding amount is regulated at a position where the normal magnetic flux density inflection point exists in the developer carrier rotating direction (i.e., where the magnetic field curve is close to the developer carrier), an insufficient developer binding force of the developer carrier 41 at the normal magnetic flux density inflection point would cause the developer holding amount to be unstable after the developer has passed the developer regulating member 42. Consequently, the thickness of the developer held on the developer carrier 41 would vary after the regulation, although a small torque required by the developer carrier 41 is preferable. Therefore, as illustrated in FIG. 3, the developer regulating member has been conventionally disposed at a position where the thickness of the developer carried on the developer carrier 41 begins to increase, i.e., at a midway position at which the normal component of the magnetic field becomes stronger, while avoiding the normal magnetic flux density inflection point. In this manner, the torque of the developer carrier 41 not being excessive is preferable, and the thickness of the developer is uniform after the regulation, thus preventing the inconveniences as mentioned above.
This positioning, however, is determined only in consideration of the developer holding amount regulated by the developer regulating member and drawn on the developer carrier, and a torque required by the developer carrier, whereas behaviors of the developer. which is regulated by the developer regulating member and returned to the developing unit, are out of consideration. Thus, with such simple positioning, the developer, which is regulated by the developer regulating member to return to the developing unit, would be pressed by the developer regulating member and separated from the developer carrier, after coming in contact with the developer regulating member, and pushed up over the surface of the developer regulating member in contact therewith. Then, the developer once drops by gravity at a point where it is free from the influence of the magnetic flux density of the developer carrier. The developer having dropped, however, is again drawn up by the developer carrier immediately. As a result, the circulation of the developer cannot progress in the entire developing unit so that the toner concentration is uneven within the developing unit, thereby causing problems such as an uneven image concentration and so on.
Conventionally, as a method of improving the capability of mixing a developer in a developing unit by stirring to eliminate an uneven toner concentration of the developer and so on, for example, as described in Japanese Laid-open Patent Publication No. JPAP09-080881 (1997), it is known to improve a developer stirring member to forcibly stir a new developer and a returned developer with the improved developer stirring member and so on to provide a uniform mixture of the developers. Disadvantageously, however, this method involves a complicated mechanism, an increase in unit torque, an increased number of parts, and so on to cause a higher cost. Therefore, such a method cannot satisfy the requirements for down sizing and energy saving.